1. Field of Invention
The field of the present invention relates in general to modems and more particularly digital modems.
2. Description of the Related Art
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. Subscriber lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud. With X-DSL significant increases in bandwidth have been made possible by utilizing frequencies higher than the voice band to deliver services such as: data, video, audio etc. Thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation.
Currently there are over ten XDSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there may be more than one line code, or modulation protocol, e.g. discrete multi-tone (DMT) and carrier less AM/PM (CAP). DMT modulation involves establishing a communication channel with a plurality of sub-channels each with a center frequency a.k.a. carrier tone. The sub-channels are frequency division multiplexed across the available bandwidth. Each sub-channel uses quadrature phase amplitude modulation (QPAM) to modulate information. The center frequency, a.k.a. tone of each sub-channel serves as the carrier on which QPAM modulation of information is effected. The information modulated on a tone is identified in the frequency domain as a sub-symbol which defines a unique phase and amplitude relationship between the carrier tone and the information modulated on it. Each sub-symbol may be expressed as a complex number. Specific bits of information are converted to a corresponding complex number using a mapping table, which defines for all possible phase and amplitudes supported by the DMT protocol corresponding binary bits. Collectively all the sub-symbols modulated on each tone across a tone set are defined as a symbol, with the symbol rate defined by the corresponding X-DSL standard.
Typically initialization of an X-DSL modem utilizing a DMT modulation protocol, begins with several discrete phases of initialization including: handshake, training and channel analysis. During initialization system clocks on opposing modems are synchronized along with symbol boundaries. This synchronization is accomplished with a fixed set of data, a.k.a. pilot tones, modulated on selected sub-channels. The pilot tones and the manner in which they are modulated are known to both sending and receiving modems. Additionally, each tone is modulated with varying amount of data in order to maximize the data carrying capability of the communication medium that links the modems. During this phase the modems pass parameters which allow each to establish a bit loading for each tone across a tone set or symbol. The outcome of initialization is a contract between the modems on communication parameters which takes into account both modem capabilities and limitations in the quality of the communication medium linking the modems.
After initialization the modems enter show time phase of operation in which high speed communication of voice, data and video takes place in the allocated upstream and downstream frequency bands over the subscriber line. During show time clock recovery typically on the customer premises equipment (CPE) is used to synchronize the high speed system clock on the CPE modem with the clock of the central office (CO) modem to which it is coupled. The CPE modem has a reference clock with approximately the same frequency as that of the CO. The CPE then phase aligns with transitions in the downstream communications from the CO using a phase-locked loop circuit (PLL). The accurate synchronization of the clocks of the CO and CPE modems is critical to the orthogonality of the sub-carriers or tones and thus to the integrity of the show time communication.
The VDSL2 standard is embodied in the International Telecommunication Union (ITU) Recommendation G.993.2 of February 2006 entitled: “Series G: Transmission Systems and Media, Digital Systems and Networks . . . Very High Speed Digital Subscriber Line Transceivers 2 (VDSL2)” In Section 10.4.5 of that standard entitled “Synchronization” the use of sixteen or fewer pilot tones is specified for providing the transitions required for clock recovery during initialization and show time phases of modem operation. These tones are selected by the CPE modem and remain fixed throughout show time operation. The modulation associated with a sub-carrier designated as a pilot tone is specified as 4-QAM (Quadrature Amplitude Modulation) with a fixed value of ‘00’.
What is needed is a modem with improved capabilities for clock recovery during show time phase of modem operation.